I try to note start and end times of activities. At work, or whenever I'm doing something on my laptop I would fill the spreadsheet as soon as I finish or take a break from an activity (eg finished reading a paper, going on a coffee break now). When I'm doing something more varied I would note start/end times via google sheets app on my phone or just remember them and fill them once I have access to my laptop. Sometimes, I would use data from my fitbit (to see when I started/stopped walking) or my search history (to see changes between working vs looking at silly things on the internet). The categories were broad enough that logging the data is not too time-consuming and often they could be added as big time-blocks of a single activity.